Flexible mountings for attachment between a supporting member and a supported member, which exhibit a high degree of roll restraint of the supported member, yet still isolate the supporting member from vibration, are desirable in some applications. A high roll stiffness comes into play when one side of the mounting at the attachment to the supported member is loaded upwardly while the other is loaded downwardly. Vertical loading of the mounting occurs when the supporting member and the attachment points to the mounting on both sides of the supported member move up or down together. A high degree of roll restraint is particularly useful to eliminate large lateral excursions or motions of the supported member when subjected to torsional moments due to side loads applied to the supported member below the mounting. This is particularly true in aircraft engine applications, where exceeding a lateral sway space envelope requirement will cause interference between the engine and the engine cowl, or between the engine and other structural components. In general, the mounting attributes which produce high roll stiffness do not result in low translational stiffness which is desirable for vibration isolation, i.e., they impose contrary requirements.
One such high roll stiffness mounting is described in U.S. Pat. No. 4,634,081 to Chee, wherein an aft engine mount with vibration isolators is described for suspending an aircraft engine core 10 below an engine mounting strut 12. The mounting includes a yoke 16 which is affixed to the ears 18 and 20, which are formed on the mounting ring of the engine. At least one mounting link 24 is used for accommodating radial engine expansion due to temperature. Spaced-apart isolator assemblies are captivated by isolator housings 32 such that engine weight is supported. In addition, this spacing of the isolator assemblies provides for a relatively high roll stiffness. The tubeform-type isolator assemblies provide effectively soft lateral stiffnesses for lateral vibration isolation. However, due to limited space, or envelope, available for locating the tubeform-type isolator assemblies far apart, the vertical stiffness of each of the isolator assemblies must be very high to obtain adequate roll stiffness, and to control motions of the supported member due to side loads. A high vertical stiffness will limit the level of isolation achievable in the vertical and pitch modes of the system. Also, it should be emphasized that this type of mounting includes at least one link to accommodate radial engine expansion and an expensive yoke attachment between the mounting and the supported member, thus requiring a large space envelope.
The commonly assigned U.S. Pat. No. 4,805,851 to Herbst, which is hereby incorporated by reference, provides a solution to the problem of how to get soft vertical and lateral stiffnesses for adequate vibration isolation, yet provide a very high roll stiffness for control of lateral engine loads. This was accomplished by providing an aft engine mounting bracket assembly 12 comprising a base 13, a torque tube shaft 22 with arms 20 and 21 extending from the ends of the torque tube 22. Further, the mounting assembly 12 included a cap 48 for attaching an elastomeric bearing 26 to the base 13. The elastomeric bearing 26 is very stiff vertically and provides a high degree of roll stiffness when arms 20 and 21 are loaded vertically in opposite directions. Spacing the engine attachment points forward of the elastomeric bearings 26 on the arms 20 and 21 makes them act as levers. Therefore, when both arms move together, the effective vertical stiffness at the engine is sufficiently low to provide superior isolation. At the same time, roll moments which occur when the arms are loaded in opposite directions, are restrained by the extremely rigid torque tube shaft 22. Again, like the Chee U.S. Pat. No. '081 mounting, this isolator assembly requires a yoke or hanger for attachment to the arms 20 and 21, and at least one link to accommodate the radial engine expansion due to thermal effects. Also, it should be noted, because of the high profile of the mount and yoke, this configuration requires a significant vertical space envelope between the supporting member and the supported member.
The commonly assigned U.S. Pat. No. 5,108,045 to Law et al. entitled "Engine Mounting Assembly", which is hereby incorporated by reference, describes an engine mounting which improves over the Herbst U.S. Pat. No. '851 concept by providing elastomer sections which are effectively loaded in pure shear or pure compression, thus leading to enhanced service life and linearity. Although the performance of this mounting is superior, it like the Herbst U.S. Pat. No. '851 and Chee U.S. Pat. No. '081 mounting concepts requires a hanger or yoke to transfer loads into the engine mount. It also requires at least one link to accommodate the radial engine expansion. In addition, like the Herbst U.S. Pat. No. '851 mounting, the height of the mount and yoke will require a significant amount of space between the supporting member and the supported member. It should be emphasized that all the abovementioned mountings have lateral elastic centers which are far above the center of gravity (CG) of the supported member. Thus, applied side loads will cause a high degree of rotation of the supported member about this elastic center, imparting a high degree of lateral motion of the CG.